1. Technical Field
The present invention generally relates to mobile communication systems and, in particular, to a method and system for detecting adjacent channel interference (ACI) in an orthogonal frequency division multiplexing (OFDM)/OFDMA based broadband wireless access system.
2. Description of the Related Art
For two communication systems that operate on adjacent frequency bands, adjacent channel interference (ACI) in the adjacent frequency bands is usually caused by unwanted emission due to the imperfectness of a transmitting filter. FIG. 1 is a diagram representation of a frequency spectrum 100 of two conventional frequency division duplex (FDD) systems using adjacent frequency bands. The uplink (UL) of the “FDD A” system is adjacent to the UL of the “FDD B” system. Likewise, the downlink (DL) of the FDD A system is adjacent to the DL of the FDD B system. A duplex separation gap is shown to extend from the beginning of the frequencies of the UL of the FDD A system to the beginning of the DL of the FDD A system. If there is only a small frequency gap between the UL of FDD B system and the DL of FDD A system, the ACI can occur between two co-located base stations (BS) and two close-by mobile stations (MS).
FIG. 2 is a block diagram representation of a frequency spectrum 200 of a FDD system and a time division duplex (TDD) system using adjacent frequency bands. The UL of the FDD A system has frequencies that are adjacent to a “TDD B” frequency band. The DL of the FDD A system is adjacent to a second TDD B frequency band. When the two systems involved are two unsynchronized TDDs or one TDD and one FDD, the ACI can be present between two co-located BSs and two close-by MSs. MS-MS interference is the most problematic type of ACI due to the mobility associated with the MSs and the size and cost limitations on the handsets. In particular, as the size of the MS decreases to achieve maximum portability, the dimension of the RF filter used in the MS is very limited. Under current filter technology, it is challenging to build such a RF filter that can achieve the required attenuation at very small frequency offset for mitigating MS-MS interference with such limited dimension. On the other hand, increasing the RF filtering performance of MS also demands additional manufacturing cost. In high user-density areas, such as train stations and coffee shops, two MSs can be close to each other (several meters) and the short distance therebetween may lead to severe performance degradation or warrant a large frequency guard band to mitigate the ACI. BS-BS interference may also require additional channel filter(s) to be installed in the BS.
To save as much spectrum as possible from being wasted as a guard band, several attempts have been made to mitigate the ACI. One example of an ACI mitigation methodology uses system coordination, which needs information on whether ACI exists or not. In the system coordination example, the system detects ACI existence using a channel edge bandpass filter for code division multiplex access (CDMA) systems. Basically, the ACI mitigation method and ACI detection procedure is designed for CDMA systems and the filter for a CDMA system needs to be specifically designed to match a chip waveform and receiver filtering.
If a signal-processing based ACI cancellation method is used, the existence of ACI needs to be determined because such methods usually cause unnecessary computational burden when the ACI does not exist.
In another example, if two systems are both TDD, time synchronization could be used to reduce the timing overlap when one system is working in an uplink cycle and the other is working in a downlink cycle. However, such a method does not apply to the case where one system is FDD and the other is TDD and the case of two FDD systems with small frequency gap between the DL block of system A and the UL block of system B as shown in FIG. 1. Moreover, to completely eliminate the MS-MS interference, the two TDD systems need to have the same downlink/uplink split and frame duration.
In yet another example, a technique to minimize inter-system MS-to-MS interference via inter-system coordination detects the existence of the ACI by tracking the signal interference-to-noise ratio (SINR) change and signal energy at the MS. However, the technique does not employ the frequency spectrum feature of ACI which usually has a higher energy at the channel edge. Therefore, the detection method does not provide reliable results.